The long-lasting earthquake swarm leading up to the 2024 M7.6 Noto-Hanto earthquake, Japan

An intense earthquake swarm has persisted for more than three years within a 20 km × 20 km area beneath the northeastern tip of the Noto Peninsula, central Japan since November 2020. The largest magnitude for each year from 2021 to 2023 increased to 5.1, 5.4, and 6.5 by the end of 2023. On January 1st, 2024, an M7.6 earthquake rupture nucleated within the swarm area and propagated bilaterally toward ENE and WSW directions along multiple faults. Globally, it is rare that the long-lasting seismic swarm preceded such a large event. We have analyzed the long-term continuous seismic waveforms to create a more precise earthquake catalog associated with this earthquake sequence. Based on this catalog, we have explored the spatial-temporal evolution of the seismicity before the M7.6 event. Note that the foreshock sequence, including a M5.7 event, started approximately 1 hour before the M7.6 event close to the hypocenter. The M7.6 nucleated from the deepest side of one of numerous planer clusters that were dominantly dipping toward the southeast direction. Several previous studies using seismic and geodetic data suggest that the long-lasting earthquake swarm has been driven by upward fluid flow along pre-existing cracks/faults in the crust (e.g., Nishimura et al. 2023). Especially, Kato (2024, doi:1029/2023GL106444) recognized a rapid upward migration of the immediate aftershocks following the 2023 M6.5 and M5.9 events and implied fault-valve behavior that might be driven by upwelling of crustal fluids along the intensely fractured and permeable fault zones via the dynamic ruptures. If fluids could migrate along pre-existing faults, fault strength would be reduced by lubrication. In addition, the long-lasting intense seismicity and slow deformations detected by GNSS network have partially released the accumulated elastic stress in the swarm area, resulting in stress loading onto nearby fault segments. The strength of the faults gradually decreased, and the stress was partially released over three years, which may have triggered the latest M7.6 earthquake.